This invention relates to instruments for measuring bending deformation in materials, and more particularly to a fiber optic interferometric method for such measurements.
Many situations exist in which it is desired to measure the deformation or orientation of structures and structural members. Examples of such situations are the monitoring of buildings and bridges for safety, feedback control of buildings for the prevention of earthquake damage, guidance and location of drills in geophysical exploration and oil production, disaster prevention in mines, tolerance maintenance in manufacturing processes, and monitoring and control in the assembly of structures.
In some cases, the environment in which the measurement is required can be somewhat hostile. For example, there may be corrosive chemicals present, or the structures may be subjected to high temperatures and pressures, or there may be electromagnetic interference. Accordingly, there is a need for a structural monitor that is chemically inert, insensitive to temperature and pressure, and operable in the presence of electromagnetic interference.
One approach to measuring structural bending is fiber optic interferometry. Fiber optic sensors make use of the phase modulation experienced by light propagating through an optical fiber that is exposed to an external environment. The phase modulation is interferometrically retrieved and processed to determine a desired characteristic of that environment. When properly configured relative to a certain material or surface, any external disturbance that affects the length of the fiber, such as strain, pressure, temperature, acoustics, or vibration, causes a phase change in the detected light signal.
An advantage to the fiber optic interferometric approach is that silica fiber is chemically inert. Also, freedom from electromagnetic interference can be achieved by placing the associated electronics remote from the monitoring site. However, sensitivity to pressure and temperature can be problematic.
Two basic techniques have been used to eliminate pressure and temperature induced changes. In one technique, an attempt is made to shield the fibers from pressure variations and to insulate the fibers from temperature changes. In the other technique, an attempt is made to monitor the pressure and temperature changes, and to subtract out these effects. For interferometric measurements of strain on extended structures, both of these techniques can be hard to implement because shielding and monitoring must be done over large expanses of material.
One aspect of the invention is a method of using two interferometric configurations to measure bending of an extended structural element. The two configurations can be two interferometers, or the same interferometer with two different measurement arms. First, a segment of a first optical fiber is attached along one side of the element, the first optical fiber comprising the measurement arm of a first interferometric configuration. A set of interference fringe values from the first interferometric configuration is obtained. Next, a segment of a second optical fiber is attached along one side of the element and another segment of the same fiber along the opposing side of the element, the second optical fiber comprising the measurement arm of a second interferometric configuration. A set of interference fringe values is obtained from the second interferometric configuration. The second set of interference fringe values is subtracted from the first set of interference fringe values, to yield a set of intensity difference values that indicates only the effects of bending and not temperature or pressure. Bending and bend-induced strain can be calculated from these values.
Advantages of fiber interferometric measurement of bending are its sensitivity, inertness to harsh chemical environments, and freedom from electromagnetic interference. Additionally, the invention is an improvement over previous fiber optic interferometric methods because the effects of temperature and pressure are intrinsically eliminated, that is, without the need for shielding or special processing. Also, because the optical fibers are attached to the beam along its length, the measurement is a global rather than a local one.